Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may employ a fixing roller inside which a heater is provided and a pressing roller pressed against the fixing roller to form a fixing nip therebetween. As a recording medium bearing a toner image passes through the fixing nip, the fixing roller heated by the heater and the pressing roller apply heat and pressure to the recording medium to fix the toner image on the recording medium.
For example, FIG. 1 is a vertical sectional view of a fixing device 27R having such configuration. As illustrated in FIG. 1, a pressing roller 200 is pressed against a fixing roller 100 to form a fixing nip N therebetween. As a recording medium P bearing a toner image T passes through the fixing nip N, the fixing roller 100 heated by a heater 300 and the pressing roller 200 apply heat and pressure to the recording medium P bearing the toner image T, thus fixing the toner image T on the recording medium P.
After passing through the fixing nip N, the recording medium P bearing the fixed toner image T should be conveyed in a conveyance direction Da. However, since toner of the toner image T is melted by the heated fixing roller 100 at the fixing nip N, it may adhere to the fixing roller 100, deviating the conveyance direction of the recording medium P from the conveyance direction Da toward the fixing roller 100. For example, the conveyance direction of the recording medium P may be angled leftward beyond a border b, and thus the recording medium P may be wound around the fixing roller 100.
Specifically, where F1 defines an adhering force of the melted toner of the toner image T which makes the recording medium P adherent to the fixing roller 100 and F2 defines a bending force required to wind the recording medium P around the fixing roller 100, that is, a force required to angle the recording medium P by an angle θ from the conveyance direction Da to the border b, when the adhering force F1 is smaller than the bending force F2, winding of the recording medium P around the fixing roller 100 can be prevented.
To make the adhering force F1 smaller than the bending force F2, the toner of the toner image T may contain a release agent, such as wax, that facilitates separation of the toner from the fixing roller 100. Conversely, the fixing roller 100 may be downsized into a fixing roller 100s having a smaller diameter to change the border defining the limit that prevents winding of the recording medium P around the fixing roller 100 from the border b to a border b′, thus increasing the bending force F2 required to wind the recording medium P around the fixing roller 100.
However, even with the above methods of decreasing the adhering force F1 of the toner of the toner image T and downsizing the fixing roller 100 to increase the bending force F2, when a recording sheet of reduced rigidity, such as a thin sheet, is used as a recording medium P, such recording sheet may decrease the bending force F2 required to wind itself around the fixing roller 100. As a result, the non-rigid recording sheet may be wound around the fixing roller 100.